Tuning friction and slip at solid-nanoparticle suspension interfaces by electric fields

Abstract

We report an experimental Quartz Crystal Microbalance (QCM) study of tuning interfacial friction and slip lengths for aqueous suspensions of TiO₂ and Al₂O₃ nanoparticles on planar platinum surfaces by external electric fields. Data were analyzed within theoretical frameworks that incorporate slippage at the QCM surface electrode or alternatively at the surface of adsorbed particles, yielding values for the slip lengths between 0 and 30 nm. Measurements were performed for negatively charged TiO₂ and positively charged Al₂O₃ nanoparticles in both the absence and presence of external electric fields. Without the field the slip lengths inferred for the TiO₂ suspensions were higher than those for the Al₂O₃ suspensions, a result that was consistent with contact angle measurements also performed on the samples. Attraction and retraction of particles perpendicular to the surface by means of an externally applied field resulted in increased and decreased interfacial friction levels and slip lengths. The variation was observed to be non-monotonic, with a profile attributed to the physical properties of interstitial water layers present between the nanoparticles and the platinum substrate.

Figure 1

Schematic of the apparatus and an example of the response of positively charged Al₂O₃ nanoparticles and negatively charged TiO₂ nanoparticles to a positive bias voltage applied to the Pt QCM electrode. Positive bias voltages repel (attract) the Al₂O₃ (TiO₂) nanoparticles away from (towards) the surface, changing both the number of nanoparticles near the surface as well as the location of the hydrodynamic slip plane and the electrical double layer. Interfacial frictional drag levels are highly sensitive to both effects.

Figure 2

(a) Schematic of transverse shear motion of QCM motion with oscillation amplitude V_q immersed in a nanoparticle suspension along with the various theoretical frameworks employed to analyze the QCM data. The models fall into three general categories: “Bulk” (b,c) uniform suspensions, “Electrolyte” (d,e) slippage occurs at the boundary of a non-slipping surface layer with the fluid, and “Adsorbed film”(f) slippage occurs at the boundary of a dense adsorbed film with the substrate.

Figure 3

(Left) Photographic images of sessile droplets of (upper) pure water, and 0.67 wt% suspensions of (middle) TiO₂ and (lower) Al₂O₃ atop the QCM platinum electrode, each with a scale bar of 0.3 cm. (Right) An AFM image of the surface topography of the electrode. For all Pt surface electrodes, both before and after an exposure to TiO₂ and Al₂O₃, the rms roughness was 1.8 ± 0.2 nm, the fractal dimension was 2.54 ± 0.1, and the correlation length was 110 ± 10 nm.

Figure 4

QCM frequency f (a) and resistance R (b) versus time as the concentration of positively (negatively) charged Al₂O₃ (TiO₂) nanoparticles is increased from 0 to 1 wt% in six equal increments. TiO₂ nanoparticles notably reduce frictional drag forces (R-R_water) while Al₂O₃ nanoparticles increase them. (c-f) QCM frequency (c,e) and resistance (d,f) versus time for static electric field conditions for 0.67 wt% nanoparticle suspensions, where t = 0 is the time at which the applied squared wave bias voltage was reversed (once per 150s for Al₂O₃ suspensions and once per 100s for TiO₂ suspensions). Error bars represent the standard deviation of five separate measurements.

Figure 5

QCM frequency (a,b) and resistance (c,d) response for 0.67 wt% suspensions of Al₂O₃ and TiO₂ nanoparticles and pure water (blue) for one cycle of applied alternating electric field (e,f). The features denoted by dashed lines (c–f) in between 1 and 1.25 V and may indicate TiO₂ nanoparticles being pressed into interfacial water molecules, which also reorient at positive applied bias voltages.